This invention concerns the atmospheric and subatmospheric treatment of particulate materials by a plasma-activated gas species to modify the particle surfaces. U.S. Pat. No. 4,478,643 discloses a pigment treated with a low temperature plasma wherein the pigment is maintained in a commercially available plasma asher for a time that is excessive and will cause pigment degradation. U.S. Pat. No. 5,234,723 discloses a process for treating particulates with a plasma-activated species at subatmospheric pressure by dropping a particulate through said treatment zone homogeneously. U.S. Pat. No. 5,176,938 discloses a process for treating particulates by passing them through a plasma it: flame. U.S. Pat. No. 5,340,618 discloses a method of treating a powder with an atmospheric pressure plasma by dispersing the powder within the reaction chamber using what appears to be a fluidized bed arrangement without addressing the problem of deagglomeration.
The process of the instant invention avoids the harsh treatment conditions of the ""643 and ""938 processes and the agglomeration of the ""723 and ""618 processes.
The organic particles contemplated for surface modification by the method of this invention have varying affinities for clumping together as agglomerates and/or aggregates by virtue of physical and/or chemical attractions among particles. It is believed that plasma activated species essentially will only interact with, and thereby treat, the surfaces of the clumps. When clumps or agglomerates are treated, only primary particles that are at or near the surface of the clump will be treated by the plasma activated species. It therefore follows that the larger the size of the clump, the smaller the proportion of the primary particles within the clump that can be treated by the gas species and hence the less effective the treatment process will be.
There is almost nothing in the prior art to guide one in the successful implementation of a modification process for materials that readily agglonmerate. In this regard, it has now been discovered that successful surface modification of particles requires an integrated process for deagglomerating clumps of particles and treating them while in their deagglomerated state and before they have an opportunity to reagglomerate. It has been found necessary for readily clumping particulate materials, to treat the particles while they are in a subdivided state, in order to achieve effective treatment at commercial production rates. The degree to which the clumps are subdivided must be no greater than a certain multiple of the mean dimension of the primary particles. The required conditions for the successful modification of normally clumping particulate organic materials by the process of this invention are as described in more detail hereafter.
This invention concerns a method for treating the surface of particulate materials that exhibit irreversible degradation when heated above a threshold temperature not exceeding about 500 degrees C. with at least one species of plasma-activated process gas in a treatment chamber that contains the plasma source or is downstream of the plasma source, comprising the steps of:
i introducing activated process gas into the treatment chamber,
ii deagglomerating the particles into fractions to expose at least a portion of the surface of substantially each fraction of deagglomerated particles,
iii contacting the deagglomerated particle fractions with the activated process gas in the reaction chamber before reagglomeration can occur to any substantial degree,
iv modifying at least a portion of the surface of substantially each deagglomerated particle fraction with the activated process gas, and
v controlling the temperature of the deagglomerated particle fractions so they do not thermally degrade.
This invention also concerns a method for grinding agglomerated pigment particles comprising the steps:
i disposing to the grinding mechanism agglomerated particles that have been surface-modified by the method of this invention, and
ii grinding said particles to effect deagglomeration, whereby the deagglomeration is effected at less than about 85% of the time it would take under identical conditions to deagglomerate pigment particles not surface-modified by the method of this invention.
This process will effect size reduction of the agglomerated particles. Wet-milling and high-shear mixing can also be employed in deagglomeration.
This invention also concerns a color composition (color concentrate, millbase, or finished paint) comprising surface-treated pigment particles, the composition having the following properties:
i viscosity less than one-half that of a like composition comprising untreated particles of the same pigment,
ii improved tinting strength at least 2% above that of a like composition comprising untreated particles of the same pigment, and
iii increased pigment to binder ratio at least about 5% above that of a like composition comprising untreated pigment.
It has been found in treating a normally difficult-to-treat pigment, whose color is readily degraded, such as red pigment, that surface treatment can be accomplished by the process of this invention causing a color shift on the L*a*b* color scale of only xc2x10.3, a color shift less than is observable by the average human observer. As used herein color measurements are expressed in the L*a*b* color space according to ASTM Standards on Color and Appearance Measurement First Edition, American Society for Testing and Materiais (ASTM). Philadelphia, 1984.
This invention also concerns an improvement in an apparatus for treating particles at atmospheric or subatmospheric pressure comprising a reaction or treatment zone in a reaction chamber having particle inlet and outlet means, a plasma-generating source either inside or outside the reaction zone, and a particle storage hopper that cooperates with the inlet. The improvement comprises:
i a particle-filled storage hopper designed to operate at a pressure above that of the reaction chamber;
ii a module located between the particle storage hopper and the inlet that cooperates with both, the module comprising means for controlling a feed rate of the particles, for deagglomerating the particles, and for introducing deagglomerated particles into the reaction zone, and
iii means for maintaining the deagglomerated particles at a temperature below that at which they degrade.
The apparatus described above will comprise a deagglomeration module that deagglomerates and disperses the particles into the reaction zone characterized by these features:
i a mass flow mechanism that moves particles from the hopper to the reaction chamber inlet at a controllable rate of particle mass per unit time, and
ii a mechanism for introducing the particles into the reaction zone in a manner to effect particle deagglomeration and dispersion.
It is preferred that the deagglomeration module comprise a mixing chamber for mixing particulates and a carrier gas, mechanical stirring means, an ultrasonic horn energized by an ultrasonic transducer, and an outlet orifice, wherein the pressure in the mixing chamber is maintained above the pressure in the reaction chamber, and wherein the region immediately adjacent the outlet orifice is ultrasonically agitated with sufficient energy to prevent plugging of the orifice by the particles and to facilitate deagglomeration of the particles, the carrier gas then carrying the deagglomerated particles into the reaction chamber. Also contemplated are other means including mechanical deagglomeration and dispersal of the particles with or without the aid of a carrier gas.
This invention also concerns an improvement in an apparatus for treating particles at subatmospheric pressure comprising a reaction or treatment zone in a reaction chamber having particle inlet and outlet means a plasma-generating source outside the reaction zone and a particle storage hopper that cooperates with the inlet. The improvement comprises:
i a particle-filled storage hopper;
ii means for controlling a feed rate of the particles from the storage hopper;
iii a module located between the particle storage hopper and the inlet that cooperates with both, the module comprising means for controlling feed rate of the particles and for deagglomerating the particles, and introducing deagglomerated particles into the reaction zone; and
iv means for maintaining the reaction zone at a low plasma potential.
This invention also concerns an improvement in an apparatus for treating particles at atmospheric pressure comprising a reaction or treatment zone in a reaction chamber having particle inlet and outlet means, a plasma-generating source either inside or outside of the reaction zone, and a particle storage hopper that cooperates with the inlet. The improvement comprises:
i a particle-filled storage hopper designed to operate at above atmospheric pressure;
ii a module located between the particle storage hopper and the inlet that cooperates with both, the module comprising means for controlling a feed rate of the particles, for deagglomerating the particles, and for introducing the deagglomerated particles into the reaction zone, and
iii means for maintaining the deagglomerated particles at a temperature below the temperature at which they degrade.